Multiple Realizability Revisited

"Anatomy first, and then physiology; but if physiology first,
then not without anatomy."

B. von Gudden, cited in Zeki, 1993, p. 72.

(from the Proceedings of the Australian Cognitive Science Society,
1997)

William Bechtel and Jennifer Mundale

Washington University and Hartwick College

e-mail: bechtel@twinearth.wustl.edu and mundalej@hartwick.edu

Abstract

The claim of the multiple realizability of mental states by brain states
has been a major feature of the dominant philosophy of mind of the late
20th century. The claim is usually motivated by evidence that mental states
are multiply realized, both within humans and between humans and other
species. We challenge this contention by focusing on how neuroscientists
differentiate brain areas. The fact that they rely centrally on psychological
measures in mapping the brain and do so in a comparative fashion undercuts
the likelihood that, at least within organic life forms, we are likely
to find cases of multiply realized psychological functions.

Hilary Putnam's original multiple realizability claim, that the same
mental state can be realized by different brain states, and/or that the
same brain state can realize different mental states, has become orthodoxy
in the philosophy of mind. He employed it to argue against a version of
the identity theory popular in the 1960s, which sought to identify mental
states with specific brain states. In sum, if the relation between mental
states and brain states is many to many--the main thrust of multiple realizability--then
no identity relation holds between mental states and brain states. One
common corollary of this rejection of the identity thesis is the contention
that information about the brain is of little or no relevance to understanding
mental processes.

When Putnam (1967) characterizes brain states he treats them as physical-chemical
states of the brain although he never tells us just what these are. Moreover,
while it may be an intuitively plausible way of characterizing brain states,
the notion of a brain state is actually something of a philosopher's fiction.
A notion closer to what neuroscientists would use is activity in the same
brain part or conglomerate of parts, thus making the identification of
these separate parts a task critical to the practice of neuroscientists.
Below, we discuss how the scientifically operative notion of a "brain state"
differs from the sort of fine-grained conception of brain states employed
in philosophy. In section 1, we focus on neuroanatomical brain mapping
research, emphasizing two noteworthy points: (1) even in neuroanatomy,
the appeal to function, especially psychological function, is an essential
part of the both the project and its tools, and (2) the cartographic project
itself is frequently carried out comparatively--across species. Yet, to
heed the call of multiple realizability, not only would brain taxonomy
have to be carried out independently of mental function, but it would also
have to proceed without comparative evaluation. We extend this line of
thinking into section 2, where we emphasize the reciprocity of the heuristic
relationship between psychological function and brain mapping research.

Since the multiple realizability argument has been so influential in
philosophical circles, in part 3, we undertake to explain why. While we
believe there are several reasons, we concentrate on two, in particular:
(1) different grain sizes have been used in the taxonomies of mental and
brain states when establishing the one-to-many and the many-to-one mappings
between them, and (2) the multiple realizability argument has been presented
in a contextual vacuum; i.e., its advocates have typically failed to provide
us with a statement of the identity conditions of mental states and brain
states to which they were implicitly appealing in asserting cases of one-to-many
and many-to-one relationships.

1. Neuroanatomical Approaches to Mapping the Brain

The task of mapping the brain has been a challenging and controversial
one. The brain does not come pre-delineated into parts, no more than does
the world's geography. Rather, investigators have to decide what kinds
of criteria to use to mark borders. In this section, we discuss the success
of some of the more straightforwardly neuroanatomical approaches, setting
aside functional criteria until the next section.

One of the most celebrated maps of the brain was produced at the beginning
of the 20th century by Korbinian Brodmann. At roughly the same time, the
neuron doctrine, or the view that nerve fibers are separable, individual
cells, was just gaining acceptance. This doctrine rested largely on the
development of selective stains, such as the Golgi stain, which also made
it possible to recognize different types of neurons in the cortex,
and to discover that the cortex generally consisted of six different layers
of neurons (which stained differentially). Also, it soon became apparent
that there were significant differences across the cortex according to
the types of cells found and the thickness of different layers. This provided
the basis for differentiating brain areas and developing maps of the cortical
surface, such as Brodmann's well-known map of 47 areas in the human brain.
Significantly, Brodmann's goal in identifying different regions of the
brain was, ultimately, to understand function; he writes: "Although
my studies of localisation are based on purely anatomical considerations
and were initially conceived to resolve only anatomical problems, from
the outset my ultimate goal was the advancement of a theory of function
and its pathological deviations." (1909/1994, p. 243). It is because they
were likely to be functionally important that Brodmann thought cytoarchitectonic
differences should matter in differentiating areas. Also, his work was
distinctly comparative, drawing from 55 species ranging over 11 different
orders of mammals, including other primates. Throughout, Brodmann used
the same numbering systems to identify what he took to be homologous areas
in the different species, arguing that there is similarity in the overall
patterns of parcellation, constancy in broader regions across species,
and persistence of individual areas. While his map was later criticized
(see Lashley and Clark, 1946), and other researchers developed somewhat
different cortical maps, Brodmann's map was widely accepted, and is still
used today as a common reference.

Many of the advances beyond Brodmann in the current day involve developing
maps of the brain at a much finer resolution than Brodmann's. For example,
working just in the visual cortex of the macaque (principally Brodmann's
areas 17, 18, and 19), David van Essen and his collaborators have differentiated
32 different processing areas (Felleman and van Essen, 1991). Just as with
Brodmann, there remains considerable uncertainty as to exactly how many
brain regions there are. Moreover, there are a variety of tools used to
identify brain regions. Felleman and van Essen single out three different
sets of criteria that figure prominently in their study: architectonics,
connectivity, and topographical organization. Architectonics, Brodmann's
tool, they note, has been useful in identifying only a minority of the
areas of visual cortex. Topographical organization, which refers to the
orderly projection of the visual field over each area, was useful in distinguishing
about half of the areas, while common connectivity patterns between cells
in one area and those in another, were useful for identifying most all
of the areas. We should emphasize that topographical organization and connectivity
are both clearly features of functional import. Connectivity is important
insofar as it provides the vehicle for information to be moved from one
processing area to another, and topography preserves the orderly arrangement
of the visual scene, as projected onto the retina, so as to allow spatial
relations in the processing area to stand in for spatial relations in the
visual scene. Lastly, we should emphasize that while the goal is to understand
human visual processing, this work has been carried out mainly in the macaque,
with the clear assumption that organizational findings in them will generalize
to humans.

Next, we will summarize a body of research which typifies the sort of
productive interactions to be had between neuroscientific research and
cognitive research when multiple consilience is assumed, and multiple realizability
eschewed. We begin with the central role of function in brain mapping research.

Franz Josef Gall's familiar phrenological maps constitute one of the
earliest functionally-based maps of the brain. While a superb neuroanatomist,
his work suffered from several bad assumptions, such as that the cranium
provided a reliable measure of the size of the underlying brain area, and
that the size of a brain region was directly proportional to its functional
effectiveness. Although phrenology ultimately failed, two important aspect
of Gall's legacy are: (1) he demarcated areas in light of psychological
functions, thus giving each region a functional characterization and (2)
he based his claims on a comparative study of skulls, thus assuming
that his mappings would hold across species.

Despite Gall's failure, other localizing approaches soon followed, most
notably, deficit studies. The locus classicus for this approach
was Paul Broca's famous research in the last century on a subject named
"Tan" (so dubbed because of the one syllable he could utter) who had suffered
damage to the third convolution of the left frontal lobe with the result
of loss of articulate speech. Another famous deficit subject was Phineas
Gage, who, after suffering a lesion in ventromedial pre-frontal cortex,
underwent severe changes in personality and behavior. In interpreting such
deficits, researchers then and now have implicitly rejected multiple realizability
between human brains; they assume that the same brain areas are subserving
the same cognitive function in all human brains, so that damage to a brain
area results in a deficit to a particular cognitive function that is performed
by that area in undamaged brains. It is also noteworthy that many of the
lesion studies that are employed to understand the operation of the human
brain are actually done on other organisms, with the results being extrapolated
to the human case. While certain obvious problems exist (e.g., that functional
deficits following brain damage do not necessitate that the damaged area
actually performed the now deficient function), many weakness in deficit
studies can be mitigated by combining them with other approaches, such
as the stimulation techniques pioneered by Gustave Fritsch and Eduard Hitzig
in 1870, by David Ferrier in 1886, and in this century, by Wilder Penfield
and other investigators. Stimulation procedures seek to identify a psychological
function with particular brain areas, and in the hands of Ferrier, especially,
was pursued comparatively.

Recently, neuroimaging (especially PET and fMRI), have provided an additional
approach to identifying functionally significant areas in the brain (see,
for example, Petersen et al., 1989). From our vantage point, four aspects
of neuroimaging research are particularly relevant to the question of multiple
realizability. First, the analysis of mental processes plays a central
role in the interpretation of activation patterns; both in the subtractive
techniques employed to hone in on the brain activity which is specific
to a given function, and in the task analysis itself, determining how best
to exemplify and tax certain mental capacities in test subjects. Second,
there are differences between individual brains; far from deterring development
of neuroimaging, this has led researchers to pursue ways to map activations
onto a common coordinate system. Third, the relatively low signal to noise
ratio, especially in PET, makes it necessary to average data across subjects,
thus canceling out individual differences and highlighting commonalities.
The fact that any results at all survive averaging as well as transformation
onto a common brain indicates a great deal of commonality--much more than
the multiple realizability arguments would have us believe. Finally, most
neuroimaging to date is performed on humans, but given the lack of detailed
neuroanatomy for humans, the results are mapped onto the more detailed
neuroanatomical analyses developed for other primates.

We now turn to examine the converse side of the interdisciplinary relationship
we have been discussing and summarize a case study drawn from visual processing
research of using differentiations discovered in neural processing to guide
cognitive decomposition. Much of the work in decomposing visual processing
has taken place at the micro-level, and can be traced back at least to
the turn of the century in the work of Brodmann, on the one hand, and Salomon
Eberhard Henschen, on the other, both of whom advanced our awareness of
specialized visual processing regions in the cortex. Today, using advanced
single-cell recording techniques, researchers such as Semir Zeki (1993)
further confirm the functional specialization of cells from different visual
regions.

In a series of landmark studies in the early 1980s Leslie Ungerleider
and Mortimer Mishkin advanced a decomposition at the level of neural pathways.
Relying largely on lesion studies conducted in monkeys by Pohl, and others,
in the early 1970s, they differentiated two main routes for processing
visual information. Given the deficits produced by the lesions, they assigned
to one of the pathways the task of analyzing "the physical properties of
a visual object (such as its size, color, texture and shape)" (Mishkin
et al., 1993, p. 20); this is the so-called "what" pathway. The functional
significance of the other route, the "where" pathway, was confirmed when
damage to a particular part of it in monkeys resulted in their inability
to select a response location on the basis of a visual landmark.

The proposed distinction between these two pathways motivated a synthesizing
proposal by Livingstone and Hubel (1988) to relate further visual processing
details, such as the distinction between magnocellular and parvocellular
processing streams through the lateral geniculate nucleus, with the what
and where systems of Mishkin and Ungerleider. For our purposes, what is
important about this research is that the idea of decomposing visual processing
into two separate processing systems was suggested by neuroanatomy and
would not likely have been proposed drawing purely on behavioral data.
Moreover, although behavioral perceptual studies with humans were used
to support the decomposition, the original work was done with various species
of monkeys - a leap made on the assumption that their cognitive processing
is likely to be substantially similar to humans.

The account of two processing system we have presented, following Mishkin
and Ungerleider and Livingstone and Hubel, while offering a novel decomposition
of the visual processing system that accounts for a great deal of the data,
turns out to be too simple. We have already mentioned above the research
of Felleman and van Essen (1991) identifying 32 different processing areas
in the macaque. Their work also shows that the different processing areas
are highly interconnected, with approximately 1/3 of the possible interconnections
realized, most of them reciprocally. Moreover, there is a substantial amount
of cross-talk between components in the different processing streams. The
evidence seems to suggest that one can identify different primary processing
streams along the general lines suggested by Ungerleider and Mishkin, but
that the distinctions are not as absolute nor as simple as first proposed.
Instead of two, largely segregated routes, it is a more accurate to say
that the processing components early in the visual system take on responsibility
for processing different kinds of information about visual scenes, and
that later areas in the system are dedicated to solving specific sorts
of problems (e.g., coordinating limb movements). This is just the sort
of decomposition that can guide further cognitive analysis of visual cognition.

3. A Diagnosis of Why Multiple Realizability Looked Plausible

Our efforts have been aimed at discrediting the assumption that multiple
realizability can be established by showing that psychological states are
in fact multiply realized across individuals or species. Given the use
of psychological function and comparative analysis in identifying brain
areas, there is little room left for identifying different realizations
of mental states. Yet, a further interesting question arises as to why
the claim of multiple realizability of mental states has been so compelling
in the first place. One reason is almost certainly that those advancing
the claim were not attending to the actual procedures by which neuroscientists
identify brain areas but rather to an intuitive view of what would constitute
sameness or difference. It seems obvious, for example, that a rat brain
is sufficiently different from a human brain so that one could never treat
their states as identical. It seems plausible to say that while there are
differences in brain states between organisms, there are still many circumstances
in which we would attribute the same cognitive state to them, thus making
them appear to be multiply realized. Despite the intuitiveness of the claim
of neural differences, our endeavor has been to show that such differences
do not prevent neuroscientists from identifying common brain areas in different
species. Accordingly, neuroscientists attempt to identify the same brain
areas and same brain processing in different organisms despite whatever
differences there are.

But there is another way we can look at this same issue. When comparing
psychological states across different individuals, psychologists also tend
to ignore differences and focus on commonalities. Likewise, philosophers
such as Putnam, who proposed comparing mental states such as hunger across
species as remote as humans and octopi have abstracted away from differences.
At anything less than a very abstract level, hunger is different in the
octopi than in humans; nonetheless, just as neuroscientists abstract away
from differences between brains in identifying brain areas and brain processes,
so do psychologists and philosophers in identifying mental states. Thus,
one diagnosis of what has made the multiple realizability claim as plausible
as it has been is that researchers have employed different grains of analysis
in identifying mental states and brain states, using a coarse grain to
identify mental states as the same across individuals and species and a
fine grain to differentiate brain states. Having invoked different grains,
it is relatively easy to make a case for multiple realizability. But if
the grain size is kept constant, then multiple realizability looks far
less plausible. One can adopt either a coarse or a fine grain, as long
as one uses the same grain on both the brain and mind side. For example,
one can adopt a relatively coarse grain, equating mental states over different
individuals or across species. If one employs the same grain, though, one
will equate activity in brain areas across species, and a one-to-one mapping
is preserved. Conversely, one can adopt a very fine grain, and differentiate
mental states between individuals, or even in the same individual over
time. If one adopts a similarly fine grain in analyzing the brain, then
one is likely to map the mental differences onto brain differences, and
brain differences unto mental differences. It is usually the case that
in apparent instances of different brain activity failing to produce apparent
differences in mental states, one has usually just not used a fine enough
grain to analyze mental states. Even when taking the functionalist's tack
of identifying mental states in terms of their input and output relations
to other states and behaviors, these inputs and outputs themselves are
open to variations in grain. The belief that it's about to rain, for instance,
might be construed broadly, as wetness-avoiding-behavior, or narrowly,
as umbrella seeking behavior.

The appropriateness of grain size, of course, depends on the reason
why one is making the comparison in the first place. This raises the next
reason why we believe that multiple realizability has been so readily accepted:
the lack of context. Whenever one asks whether two items are the same or
different, the question makes little sense unless one asks about sameness
or difference with respect to some other consideration. A human's mental
state and that of an octopus might well be counted as the same in so far
as they are associated with some general feature (such as food-seeking
behavior, in the case of hunger). But with respect to other considerations,
such as how one seeks the food, what foods are sought, and under what conditions,
etc., the food-seeking behavior is different; correspondingly, there would
then be a difference in mental state. This much seems simple and apparent,
but the assertion that what we broadly call "hunger" is the same mental
state when instanced in humans and octopi has apparently been widely and
easily accepted without considering what contexts would make such equation
of mental states reasonable. If we consider the context, and keep it fixed
in doing comparative analysis both psychologically and neurally, then it
is far less likely that we will come up with genuine cases of multiple
realization.

One possible context which could inform the discussion of sameness and
difference within mental states and brain states is the one which we have
focused upon here, that invoked in neuroscientific research into brain
taxonomy. It is clear that neuroscientists define the context broadly,
and as a result have found relevant similarities in brains across species,
as well as in different brains within the same species. Two consequences
of this choice of wide grain is that it has enabled researchers to make
powerful predictions about the cognitive effects activity (or lesions)
in a given area will have across individuals and species, and has begun
to foster enhanced understanding of the information processing components
that underlie behavior.

4. Conclusions

We urge that while multiple realizability sounds intuitively plausible,
it nonetheless constitutes a bad wager. Evidence that psychological functions
are in fact multiply realized is not likely to be forthcoming given the
practices of neuroscientists. Betting on multiple consilience instead allows
for the fruitful use of neuroscience in guiding our understanding of cognitive
systems. For some theorists, multiple realizability is an essential part
of functionalism, thus suggesting that the fall of multiple realizability
would take functionalism down with it. We do not see this as inevitable;
even if one can identify mental states with activity in brain areas, that
does not render the functional characterization of mental states any less
important. As we have emphasized, the identification of brain areas cannot
even proceed without the guidance of functional characterization.